Many play or sports balls, such as tennis balls, are ‘pressurized’ by a gas filling and lose their bounce over time and use, as a result of the gas escaping from the ball shell.
Tennis balls are commonly made by gluing together respective rims of two molded rubber half shells; heating in a match mold to fuse the rims permanently together as a unitary member; injecting air or gas under pressure; gluing two (dumb-bell shape) felt pieces to the exterior in covering relation, applying heat fusible tape along a seam between the felt pieces; and, reheating in a match mold to fuse the tape with the adjacent edges of the felt pieces.
There have been numerous prior attempts over many years to perfect ball structures to obviate or reduce such disadvantageous loss of bounce, for example by so-called pressureless ball structures. One such approach dispenses with the traditional resilient (rubber) shell or hollow core entirely, relying instead on a solid core of a resilient foam composition. Another approach teaches a pressureless ball made by replacing/augmenting the rubber of the shell or hollow core with more durable/resilient compositions. In a further approach, a resiliently compressible foam core fills the shell by continued expansion within the shell itself.